1. Field of the Invention
The present invention relates to a full color image forming apparatus that uses an electrophotographic method.
2. Description of the Related Art
In a conventional image forming apparatus, a toner image formed on an image carrier (hereinafter, “photosensitive member”) is transferred onto a transfer sheet. The toner image is fixed on the transfer sheet by a fixing device by application of heat and pressure. In a full color image forming apparatus, the toner images of yellow, magenta, cyan, and black color formed on different photosensitive members are sequentially transferred on to the transfer sheet held by a transfer sheet carrier such as a transfer drum, after the toner images are developed. The transfer sheet is then removed from the transfer sheet carrier and the toner images thereon are fixed by the fixing device by application of heat and pressure.
In another full color image forming apparatus, instead of transferring the toner images onto the transfer sheet carried by the transfer sheet carrier, the four toner images superposed on an intermediate transfer member are batch-transferred on to the transfer sheet, and then fixed by the fixing device. The image forming apparatus that uses the intermediate transfer member is gaining ground due to increased printing speed enabled by transfer and superposition on the transfer sheet of all the toner images from a plurality of photosensitive members provided for each color and disposed in a row, by the time the intermediate transfer member has completed one rotation. The image forming apparatus using the intermediate transfer member does not require a transfer sheet carrier, and thus can support a wide range of transfer sheets from a thin paper (40 g/m2), a thick paper (200 g/m2), a post card, an envelope, etc. The intermediate transfer member is generally formed in a form of a drum or a belt. The disadvantage of the image forming apparatus using the intermediate transfer member is that if the image position of each color shifts, the reproduced color image also manifests a color shift, resulting in improper image reproduction.
As a measure against such color shift, a color shift detection pattern for each color is formed on the photosensitive member of an image forming unit of the respective color. These patterns are then transferred onto the intermediate transfer belt and are read by pattern detector. The color shift in a main scanning direction and a sub-scanning direction are detected based on output signals of the pattern detector and a feedback is given to a mechanical optical system including a mirror. Thus, the image writing position and the color shift due to a skew in the image position in the main scanning direction can be corrected for each image forming unit. Thus, it is possible to accurately position the image of each color. The pattern detector includes a light emitter and a light receiver. When the light from the light emitter is reflected off the intermediate transfer belt and enters the light receiver, the light is not reflected by the pattern transferred from the image carrier of the respective image forming unit. Therefore, a shift in the pattern position is detected by measuring the timing of the output signal from the light receiver.
For example, Japanese Patent No. 2609643 discloses a technology in which a memory unit that sequentially stores a registration mark image read by the pattern detector.
Japanese Patent No. 2659191 discloses a technology in which the registration marks formed on the belt and a mark calibrated on the belt are detected, and image shift correction is based on the magnitude of shift between the two marks.
Japanese Patent Laid-Open Publication No. H01-167769 discloses a printer in which space is efficiently used. In this printer, a density pattern image is transferred onto the transfer belt so that a stable color reproduction is obtained, and a process condition of each image forming station is controlled by optically measuring the density of the density pattern image. A detector to detect density of the density pattern image is configured to also function as a detection system to optically detect positioning patterns required for correcting color shifts.
However, none of the above disclosures provide satisfactory measures to ensure a stable distance between the belt and the detector or prevention of deformation of the belt during rotation, which are essential for a high degree of precision in pattern detection.
Japanese Patent Laid-Open Publication No. 2000-214693 discloses a technology in which a belt regulating member is provided near the detector.
While color shift correction or image density adjustment control, which takes a specific amount of time, the image forming apparatus cannot output images, cutting into productive time. Therefore, full color image forming apparatus that takes the shortest possible time for shift correction and adjustment is demanded.
Therefore, it is preferable that the pattern detector is located as close to the photosensitive member as possible. The pattern transferred onto the belt from the photosensitive member thus reaches the pattern detector and the pattern reading can be completed in a short time.
However, since a regulating member that prevents deformation of the belt is also close to the photosensitive member, the pattern detector becomes closed to bias rollers, over which the belt is tautly wound. If the pattern detector is closed to the regulating member to the bias rollers, the bias applied by the bias rollers during transfer of toner image or transfer sheet separation, leak occurs with respect to the regulating member. As a result, a noise that causes malfunction of the image forming apparatus itself, void, poor transfer sheet separation, and poor discharge due to temporary weakening of the electric field caused by the leak can occur.